Fig.20.2 :1,apical;2,posterior;3,anterior;4,lateral(superior);5,medial(inferior);6,superior;7,medialbasal;8,anteriorbasal;9,lateralbasal;10,posteriorbasal.Themedialbasalregions(7)arelocatedintheupperregionoftheposteriorbasalregions(10)in
Fig.20.2 .Upper lobe Middle lobe Lower lobe Upper lobe Lingula Thyroid cartilage Cricoid cartilage Trachea Left main stem bronchus Right main stem bronchus 12 3 4 8 9 10 5 1 2 3 4 8 77 6 6 9 10 5 •Fig. 20.4 Illustration of Conducting Airways and Alveolar Units of the Lung. Therelativesizeofthealveolarunitisgreatlyenlarged.Numbersatthebottomindicatetheapproximatenumberofgenerationsfromtracheatoalveoli,whichmayvaryfromasfewas10toasmanyas23.(FromWeibelER.Morphometry of the Human Lung. Heidelberg:Springer-Verlag;1963.) with the respiratory bronchioles to the alveoli are where all gas exchange occurs. This region is only approximately 5 mm long, but it is the largest volume of the lung, at a volume of approximately 2500 mL and with a surface area of 70 m2 when the lung and chest wall are at the resting volume (see
Table 20.1
The alveoli are polygonal in shape and approximately 250 µm in diameter. Alveolar spaces are responsible for most of the lung’s volume; these spaces are divided by tissue known collectively as the interstitium. The interstitium is composed primarily of lung collagen fibers and is a space in which fluid and cells can potentially accumulate. An adult has approximately 5 × 108 alveoli (
Fig. 20.6 ), which are composed of type I and type II epithelial cells. Under normal conditions, type I and type II cells exist in a 1 : 1 ratio. The type I cell occupies 96% to 98% of the surface area of the alveolus, and it is the primary site for gas exchange. The thin cytoplasm of type I cells is ideal for optimal gas diffusion. In addition, the basement membrane of type I cells and the capillary endothelium are fused, which minimizes the distance for gas diffusion and thereby facilitates gas exchange.
Type II cells are cuboidal and usually found in the “corners” of the alveolus, where they occupy 2% to 4% of the surface area. During embryonic development, the alveolar epithelium is composed entirely of type II cells, and only very late in gestation do they differentiate into type I cells and form the “normal” alveolar epithelium for optimal gas exchange. Also, type II cells synthesize pulmonary surfactant (see ), which reduces surface tension (nonsticking properties) in the alveolus and thereby promotes less resistance during inhalation and exhalation. Gas exchange occurs in the alveoli through a dense mesh-like network of capillaries and alveoli called the alveolar-capillary network. The barrier between gas in the alveoli and the red blood cell is only 1 to 2 µm thick and consists of type I alveolar epithelial cells, capillary endothelial cells, and their respective basement membranes. O2 and CO2 passively diffuse across this barrier into plasma and red blood cells (see
Chapter 24). Red blood cells pass through the network in less than 1 second, which is sufficient time for CO2 and O2 gas exchange.
Circulatory Systems in the Lung